<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            height: 2000px;
            width: 800px;
            background-color: aquamarine;
            position: relative;
            margin: 0 auto;
        }

        .son {
            height: 200px;
            width: 200px;
            background-color: black;
            position: absolute;
            left: 50%;
            margin-left: -100px;
        }
    </style>
</head>

<body>
    <div class="box">
        <div class="son"></div>
    </div>
</body>

</html>